Electrically driven vehicles such as forklifts and the like require drive systems which are capable of delivering high start-up and low speed torques for accelerating the vehicle, propelling loads up grades, creeping under load, etc. Series-wound DC motors are capable of generaling very high start-up and low speed torques. For this reason, the traditional drive motor of choice for most electrically powered vehicles has been the series-wound DC motor. Nonetheless, series-wound DC motors have a number of recognized characteristics which are not advantageous. For instance, the speed of a serious-wound DC motor will vary significantly with a load. In addition, reversing, braking and field weakening a Series motor requires robust contactors which are capable of handling full armature current. Such contactors are expensive to install and maintain.
An alternative to the series-wound DC motor is the separately excited DC motor. Separately excited motors do not suffer from the disadvantages of the series-wound DC motor but they do not provide the advantage of high start-up and low speed torques which the series-wound motor is capable of generating.
It has been suggested that series-wound motors be operated in a separately excited fashion in which the field and armature currents are independently controlled. Hong and Park in an article entitled "Microprocessor-Based High-Efficiency Drive Of A DC Motor" IEEE Transactions on Industrial Electronics, Vol. IE-34, No. 4, November, 1987, discuss the operation of series and separately excited electric motors using a microprocessor based control algorithm wherein a table look up for an optimum field current to armature current ratio is based on a function of motor rotational speed. In a DC machine, there are many combinations of field and armature current which will provide a desired motor speed and torque. One combination of field and armature current will produce the desired motor speed and torque most efficiently. It has been proposed that a constant ratio "k" of field to armature current can be used to determine the optimum field and armature currents. Hong and Park demonstrate that "k" should vary as a function of motor rotational speed, but should not be related to load. In accordance with their teachings therefore an optimal controller will derive "k" as a function of motor speed.
U.S. Pat. No. 5,039,924 which issued Aug. 13, 1991 to Avitan teaches a system for optimizing control of separately excited DC motors whereby optimization is achieved through microprocessor-based independent PWM control of a chopper (armature) and an H-bridge (field). The disadvantage of this system is that separately excited motors require much higher armature currents when operating above rated torques, which is necessary for achieving the peak torques required for most electrically powered vehicle applications. High armature currents result in commutator and brush wear because power loss follows the ratio T.sup.2 R, so that when current doubles, resistive losses quadruple which leads to premature and excessive wear on the commutator and brushes of the separately excited motor.
U.S. Pat. No. 4,730,151 which issued Mar. 8, 1988 to Florey et al. describes a method for operating an electronic control system for an operator controlled, electrically driven vehicle wherein the motor is operated in one of a series and a separately excited mode. The series mode of operation is selected for providing high start-up torques while the separately excited mode of operation is selected for providing efficient high speed operation. The disadvantages of this control system include an energy inefficient SCR chopper circuit, inefficient contactors for switching from one mode to another and no capability for regeneratively braking at low motor rotational speeds.
Traditional prior art controllers commonly use a braking technique known as "plug braking" in which the momentum of an electrically powered vehicle is braked by reversing the direction of the field current with respect to the motoring direction (direction of rotation of the armature). "Plug braking diodes" connected in parallel with the armature of a motor connected to such controllers provide a short circuit for the armature output when the field current is reversed. Because the plug braking diode is forward biased during plug braking, the controller is chopping directly from the battery into the motor field, resulting in momentarily large field currents which yield a very strong braking response. Although plug braking is very effective in overcoming momentum, it actually wastes battery current. Regenerative braking, on the other hand, replaces a part of the battery current used in building momentum. It therefore is desirable to use regenerative braking whenever possible.
A disadvantage of prior art regenerative braking techniques, such as taught in U.S. Pat. No. 4,730,151, is that they do not teach a method for providing effective regenerative braking at low motor rotational speeds. They are therefore incapable of effectively regeneratively braking momentum of an electrically powered vehicle once the speed of a vehicle has dropped below a certain limit.